This invention relates to the production of coated abrasives engineered to have patterned surfaces with properties specific to the desired application.
The proposal to deposit isolated structures such as islands of a mixture of a binder and abrasive material on a backing material has been known for many years. If the islands have very similar heights above the backing and are adequately separated then, (perhaps after a minor dressing operation), use of the product will result in reduced surface scratching and improved surface smoothness. In addition the spaces between the islands provide a route by which swarf generated by the abrasion can be dispersed from the work area.
In a conventional coated abrasive, investigation of the grinding surface reveals that a comparatively small number of the surface abrasive grits in an active abrading zone are in contact with the workpiece at the same time. As the surface wears, this number increases but equally the utility of some of those abrasive grits may be reduced by dulling. The use of abrasive surfaces comprising a uniform array of isolated islands has the advantage that the uniform islands wear at essentially the same rate such that a uniform rate of abrasion can be maintained for longer periods. In a sense the abrading work is more evenly shared among a larger number of grinding points. Moreover since the islands comprise many smaller particles of abrasive, erosion of an island uncovers new, unused abrasive particles which are as yet undulled.
One technique for forming such an array of isolated islands or dots that has been described is that of the rotogravure printing. The technique of rotogravure printing employs a roll into the surface of which a pattern of cells has been engraved. The cells are filled with the formulation and the roll is pressed against a surface and the formulation in the cells is transferred to the surface. Normally the formulation would then flow until there was no separation between the formulations deposited from any individual cell. Ultimately a layer of essentially uniform thickness would be obtained. By way of illustration, comparative Examples C and D of U.S. Pat. No. 5,152,917 describe a process in which the pattern obtained by a rotogravure process quickly lost all separation of the individual amounts deposited from the cells.
In U.S. Pat. No. 5,014,468 a binder/abrasive formulation was deposited from rotogravure cells on a roller in such a way that the formulation was laid down in a series of structures surrounding an area devoid of abrasive. This is believed to be the result of depositing less than the fill volume of the cell and only from the perimeter of each cell, which would leave the ring formations described.
The problem with the rotogravure approach has therefore always been the retention of a useful shape to the island. To formulate an abrasive/binder mixture that is sufficiently flowable to be deposited and yet sufficiently non-flowable such that it does not slump to an essentially uniform layer coating when deposited on a substrate has proved very difficult.
Chasman et al.,in U.S. Pat. No. 4,773,920 disclosed that using a rotogravure coater, it is possible to apply a uniform pattern of ridges and valleys to the binder composition which, when cured, can serve as channels for the removal of lubricant and swarf. However beyond the bare statement of possibility, no details are given that might teach how this might be carried out.
In U.S. Pat. No. 4,644,703 Kaczmarek et al. used a rotogravure roll in a more conventional fashion to deposit an abrasive/binder formulation to deposit a layer that is then smoothed out before a second layer is deposited by a rotogravure process on top of the smoothed-out first layer. There is no teaching of the nature of the final cured surface.
Another approach has been to deposit the abrasive/binder mixture on a substrate surface and then impose a pattern comprising an array of isolated islands on the mixture by curing the binder while in contact with a mold having the inverse of the desired patterned surface. This approach is described in U.S. Pat. Nos. 5,437,754; 5,378,251; 5,304,223 and 5,152,917. There are several variations on this theme but all have the common feature that each island in the pattern is set by curing the binder in contact with a molding surface.
Yet another approach is described in U.S. Pat. No. 5,863,306 in which a pattern is embossed on a surface of a layer comprising a radiation-curable binder having abrasive particles dispersed therein after the surface of the layer has been modified to increase its viscosity but before curing of the binder has been initiated.
The present invention presents a technique for tailoring the formulation formed into patterns to generate grinding properties that vary with the degree of wear the coated abrasive has experienced.
The present invention therefore provides a flexible and effective route for the production of products uniquely suited to a specific task such that multiple abrading/fining/polishing operations can be avoided.